1. Field of the Invention
The present invention relates to the use of chirped (aperiodic) quasi-phase-matched (QPM) gratings as dispersive components in ultrashort-pulse laser systems.
2. Description of the Related Art
Chirped pulse amplification (CPA) systems are well known in the field of ultrafast optics. See, for example, D. Strickland et al., Compression of Amplified Chirped Optical Pulses, Opt. Commun. Vol. 56, pp. 219 (1985). CPA techniques are used to stretch ultrashort optical pulses prior to amplification when it is necessary to reduce the unacceptably-high pulse peak powers in the amplifier material or optical components in order to avoid detrimental effects. The ultrashort pulse duration must be restored after amplification using a pulse compressor.
Pulse stretching and compression can be achieved by introducing dispersion. Group velocity dispersion (GVD) of an optical pulse arises from the difference in the group velocities for different spectral components constituting the pulse. An optical element producing this dispersion is called a dispersive delay line (DDL). A variety of dispersive delay lines are known in the prior art associated with the field of ultrashort pulse optics, where they are commonly used for ultrashort pulse stretching and compression. Optical pulses are generally considered to be ultrashort if their duration is in the range from approximately 10.sup.-15 to 10.sup.- seconds.
Conventional dispersive delay lines include: diffractive (diffraction grating based), refractive (using prisms or other refractive shapes), interferometric (using Gires-Tournois or Fabry-Perot etalons), and Bragg reflection (fiber gratings, chirped mirrors) DDLs. All of the previously known CPA systems use conventional dispersive delay lines for stretching and recompression, and all of the conventional DDLs produce dispersed output pulses at the same wavelength as the input pulses.
Diffraction gratings can provide a large amount of stretching and compression and possess a broad spectral bandwidth. However, diffractive DDLs are disadvantageous because they are typically large in size, consist of multiple components, require complicated alignment procedures and induce relatively high losses on the dispersed optical pulse. Additionally, diffractive DDLs provide a limited degree of control of the linearity of the GVD characteristics.
Refractive DDLs are advantageous because of their low loss. However, refractive DDLs provide only a small amount of stretching/compression, are relatively large in size and provide only limited control of the linearity of the dispersion characteristics.
Interferometric DDLs are compact and have low loss. However, the practical use of interferometric DDLs is very limited due to the small bandwidth and the small amount of dispersion they provide.
Advantages of Bragg reflection DDLs, such as fiber gratings, include their small size, large dispersion, large bandwidth, and arbitrary control of dispersion characteristics. The primary disadvantages of fiber gratings are the limitations on the recompressed pulse energy and insertion losses due to the reflective configuration of such DDLs. Chirped mirrors, on the other hand, have a substantially higher energy threshold for pulse distortions, but provide a very small amount of dispersion.
It is known in the prior art that unmatched, higher-order dispersion terms can arise either if the pulse stretcher and pulse compressor are different types of DDLs or if the large amount of linear GVD introduced by the material in an optical path of propagating pulses causes a dispersion mismatch between pulse stretchers and compressors of the same type. See, for example, B. E. Lemoff et al., Quintic-phase-limited, spatially uniform expansion and recompression of ultrashort optical pulses, Opt. Lett., Vol. 18, pp. 1651 (1993); S. Kane et al., Grating Compression of Third-Order Material Dispersion in the Normal Dispersion Regime: Sub-100-fs Chirped-Pulse Amplification Using a Fiber Stretcher and Grating-Pair Compressor, IEEE J. of Quantum Electron. QE-31, pp. 2052 (1995).
In general, use of DDLs for commercially viable ultrashort pulse lasers, amplifiers and pulse shapers requires improvement of certain characteristics of existing DDLs, such as compactness, manufacturability, cost, amount of loss, and ability to control the linearity of dispersion characteristics, while maintaining the key properties of large dispersion capability, large spectral bandwidth, and the capability to sustain high pulse energies. As the above description of the properties of conventional DDLs reveals, there generally exists a trade-off between the properties of any conventional DDL. Therefore, a novel device capable of providing all of the required properties in one element would be highly beneficial.
In general, solid-state mode-locked lasers operating in a chirped pulse mode are capable of obtaining higher pulse energies. An example of a solid-state laser producing increased pulse energies for chirped output is disclosed by C. Spielmann et al. in Experimental study of additive-pulse mode-locking in an ND:glass laser, IEEE J. of Quantum Electron., Vol. 27, pp. 1207 (1991).
Another example of using a dispersive delay line in an ultrafast laser system is disclosed by L. E. Nelson et al. in Efficient frequency doubling of a femtosecond fiber laser, Opt. Lett., Vol. 21, pp. 1759 (1996), where a refractive prism-based compressor is used to compress chirped pulses from a stretched-pulse additive-pulse mode-locked fiber laser prior to frequency doubling of these compressed pulses in a conventional birefringence phase-matched nonlinear crystal. Such a fiber laser can generate pulses with energies that are approximately an order of magnitude higher than those from a typical mode-locked fiber laser directly producing bandwidth-limited pulses. However, the necessary external compressor substantially increases the complexity and size of the laser system.
A dispersive delay line in an ultrafast system can be used not only for pulse stretching or compression but also for arbitrary pulse shaping, as often required for various applications in chemistry, optical communications, etc. One example of such an arrangement is disclosed by A. Weiner et al. in Programmable femtosecond pulse shaping by use of a multielement liquid-crystal phase modulator, Opt. Lett., Vol. 15, pp. 326 (1990). In such an apparatus, the pulse to be shaped is spectrally and spatially dispersed using, for example, a grating pair or a pair of prisms. The spectrum is propagated through a spacial mask which spectrally filters both the amplitude and phase of the pulse. The spectral components are then recollimated into a beam by a second grating or pair of prisms forming a reshaped optical pulse. In principle, by a proper choice of spectral mask, any required optical waveform can be generated. The main disadvantage of this method is that the apparatus is a complex system with relatively large dimensions.
Methods of quasi-phase-matching offer broad engineerability to the phasematching properties of frequency conversion devices but have, to date, almost exclusively been applied only with periodic gratings. See, for example, M. Fejer et al., Quasi-Phase-Matched Second Harmonic Generation: Tuning and Tolerances, IEEE J. of Quantum Electron. QE-28, 2631 (1992). Such use of periodic QPM has allowed interactions in wavelength ranges and use of large nonlinear coefficients not available with birefringence phasematching. While aperiodic QPM gratings were suggested by T. Suhara et al. in Theoretical Analysis of Waveguide Second-Harmonic Generation Phase Matched with Uniform and Chirped Gratings, IEEE J. of Quantum. Electron. QE-26, 1265 (1990) for the additional advantage of increasing the wavelength acceptance bandwidth in continuous-wave second harmonic generation, the important implications of the phase response of these aperiodic QPM structures to ultrashort-pulse frequency conversion have not been known or used in practice.
In U.S. patent application Ser. No. 08/824,032, Arbore et al. disclose that a quasi-phase-matched (QPM) second harmonic generator (SHG) with QPM period chirped along the crystal length, in effect, provides group velocity dispersion (GVD) at the second-harmonic wavelength. This property allows construction of unique devices for simultaneous second-harmonic generation and temporal stretching or compression of the second-harmonic output with respect to the fundamental input pulses.
The properties of this type of dispersive element provide a number of advantages over conventional dispersive delay lines. Aperiodic QPM gratings combine compact size, manufacturability and low cost, monolithic design. Further, aperiodic QPM gratings have low loss at the fundamental wavelength, and pulse energies can be scaled by scaling the beam size. Also, such QPM gratings allow nearly arbitrary control of dispersion characteristics. In addition, aperiodic QPM gratings possess the unique property of simultaneously performing SHG and pulse compression in a single device.
All of the above-mentioned articles, patents and patent applications are hereby incorporated herein by reference.